J. V. Elsden—LIgneous Rocks of Lleyn, N. Wales. 307 
completely between crossed Nicols. There isa good deal of strongly 
dichroic hornblende, sometimes intergrown with the augite, at other 
times in detached crystals. It generally polarises more brilliantly 
and extinguishes more thoroughly than the augite. Viridite patches 
are abundant, and a little secondary quartz occurs. Most of the iron 
oxide shows the grey decomposition product of ilmenite, but some 
pyrites is to be noticed. The same rock near Llangian is much more 
decomposed, the felspars being quite opaque and much viridite and 
opaque ferric matter present. Some of the viridite tracks appear to 
have outlines of augite crystals; one looking like an orthodiagonal 
section gave an angle of 135°, which nearly corresponds to the angle 
between oo P and ow P®o (clinopinacoid) in augite. Small quartz 
fissures traverse the rock. 
We now pass to the large patch coloured as porphyry on the 
Survey Map, lying between Llanbedrog and Capel Ceidio. The 
southern portion of this area consists chiefly of quartz-felsite, and at 
Pig Street, Mr. Tawney described a volcanic ash. I find a coarse 
tuff on the south side of Mynydd Mynytho, and a similar kind of 
rock to the north of Madryn, at Y Gledrydd. My other specimens 
from this area do not differ essentially from those already described 
in Woodwardian Laboratory Notes." 
The Boduan mass of porphyrite is described from several localities 
in Mr. Tawney’s paper. J will only add one more description of a 
rock taken from the extreme south side, near the village of Boduan. 
The rock here is much decomposed; the ground-mass is micro- 
crystalline, consisting of small felspar prisms, which from the small 
extinction angle is possibly oligoclase. Precise determination of the 
larger felspars, which occur porphyritically, is difficult owing to 
decomposition, but no orthoclase can be recognized with certainty. 
There is some glassy base between the crystals. A few magnetite 
grains and a good deal of opaque ferric matter occur throughout. 
No trace of hornblende remains, although a few teebly dichroic 
chloritic pseudomorphs are to be seen. This rock differs, therefore, 
from that of Carn Boduan. 
Approaching Nevin the character of the rock changes considerably. 
I give a description of the rock near Nevin, which much resembles 
that described by Mr. Tawney as probably coming from Moel Gwyn, 
and which he classes as epidiorite. The large felspars are much 
decomposed, but are apparently triclinic: they are generally filled 
with a fine granular substance, and sometimes crowds of colourless 
microliths are visible with crossed Nicols. The ground-mass consists 
chiefly of quadrangular sections, with nearly parallel extinction, 
corresponding to oligoclase. Hornblende is present in detached 
crystals, which gradually lose their fibrous structure and dichroism 
and pass into chloritic pseudomorphs. A few remnants of augite 
are visible. Magnetite is abundantly associated with the decom- 
posing hornblende, and there are some opaque rhombohedral crystal 
of ilmenite. 
1 GroLtocicaL Macazing, Vol. X. p. 70. 
